Catalytic converters reduce exhaust gas emissions in vehicles using an internal combustion engine. A three-way catalytic converter includes a substrate with a coating of catalyst materials that stimulate the oxidation of hydrocarbon and carbon monoxide and the reduction of nitrogen oxides in the exhaust gas. The catalysts operate optimally when the temperature of the catalysts is above a minimum level and when the air/fuel ratio is stoichiometric. Stoichiometry is defined as an ideal air/fuel ratio, which is 14.7 to 1 for gasoline. An air/fuel ratio referred to as “rich” is typically associated with a ratio less than stoichiometric. Likewise an air/fuel ratio referred to as “lean” is typically associated with a ratio greater than stoichiometric.
In one vehicle configuration, first and second oxygen sensors are located in a vehicle exhaust. The first oxygen sensor is positioned in an upstream location relative to the catalytic converter. The second oxygen sensor is positioned in a downstream location relative to the catalytic converter. These oxygen sensors measure the oxygen content of the exhaust. In general, the efficiency of a catalytic converter is based on its ability to hold a charge of oxygen for a given period of time.
One method of determining the efficiency of the catalytic converter is to measure the time difference it takes for the oxygen content in the exhaust to reach a predetermined threshold in response to a commanded air/fuel ratio. The efficiency of a catalytic converter is proportional to this time difference. More specifically, as the time difference increases, the ability of the catalytic converter to hold a charge of oxygen increases.
In one approach, a lean air/fuel ratio is commanded. The downstream oxygen sensor is monitored and the time difference between the commanded time and the threshold time is determined. This time difference is considered to determine the ability of the catalytic converter to hold a charge of oxygen.